At present, a contact charging method has been put into practical use as one of methods for charging the surface of an electrophotographic photosensitive member electrostatically.
The contact charging method is a method in which a voltage is applied to a charging member disposed in contact with the electrophotographic photosensitive member, to cause micro-discharge at the part of contact between the charging member and the electrophotographic photosensitive member and the vicinity thereof to charge the surface of the electrophotographic photosensitive member electrostatically.
As the charging member for charging the surface of the electrophotographic photosensitive member electrostatically, from the viewpoint of sufficiently securing a contact nip between the electrophotographic photosensitive member and the charging member, what is common is one having a support and an elastic layer (conductive elastic layer) provided on the support.
The elastic layer (conductive elastic layer) often contains low-molecular weight components in a relatively large quantity, and hence such low-molecular weight components may bleed out to contaminate the surface of the electrophotographic photosensitive member. In order to control this contamination due to bleed-out, it is also prevalent to provide on the conductive elastic layer a surface layer different therefrom and having a lower modulus of elasticity than the conductive elastic layer.
As the shape of the charging member, what is common is the shape of a roller. Hereinafter, the roller-shaped charging member is also called “charging roller”).
Of the contact charging method, a method having come into wide use is a method in which a voltage formed by superimposing an alternating-current voltage on a direct-current voltage is applied to the charging member (hereinafter also “AC+DC contact charging method”). In the case of the AC+DC contact charging method, a voltage having a peak-to-peak voltage that is twice or more the voltage at which the charging is started is used as the alternating-current voltage.
The AC+DC contact charging method is a method by which stable charging in a high charging uniformity can be performed because of the use of the alternating-current voltage. However, insofar as an alternating-current voltage source is used, this method brings about a charging assembly and an electrophotographic apparatus which are large in size and a rise in cost, compared with a method in which a voltage of direct-current voltage only is applied to the charging member (hereinafter also “DC contact charging method”).
That is, the DC contact charging method is superior to the AC+DC contact charging method in respect of making the charging assembly and electrophotographic apparatus small-sized and achievement of cost reduction.
Japanese Patent Application Laid-open No. 2004-210857 (Patent Document 1) discloses production of an elastic material having superior surface properties and releasability and having a low hardness and a heat resistance. More specifically, a solution of an organosilicon compound having at one terminal or both terminals a functional group or groups capable of reacting with a metal alkoxide is heat-treated to remove its water content and low-molecular weight components, the metal alkoxide is added to the organosilicon compound solution thus heat-treated, to prepare an organic-inorganic hybrid sol, then the sol is heated into a gel, and a substrate is, e.g., coated with the resultant organic-inorganic hybrid material to produce the elastic material having superior surface properties and releasability and having a low hardness and a heat resistance. This elastic material is useful as a material for roll members and belt members of copying machines and printers of electrophotographic systems, as so disclosed.
Japanese Patent Application Laid-open No. 2000-267394 (Patent Document 2) discloses a charging member which is brought into contact with a charging object and with which the charging object is electrostatically charged by applying a voltage across the charging member and the charging object. In this charging member, at least its member surface coming into contact with the charging object is formed of a surface layer having a binder and an additive added thereto which has a fluorine block copolymer or silicon block copolymer having a first block of a fluorine type or silicon type and a second block containing neither fluorine nor silicon. This charging member further has low friction properties and superior toner adhesion properties and besides exhibit a superior running performance, as so disclosed.